1. Field of the Invention
This invention relates to an aqueous coating composition based on specific two-component polyurethanes and to a process for its production.
2. Description of the Prior Art
Ecological factors play an important part in surface technology. A particularly urgent problem is to reduce the organic solvents used for paints and coating compositions.
Previously, it has not been possible to prepare the highest quality chemically cross-linking polyurethane paints without organic solvents. These paints by virtue of their outstanding properties have acquired considerable significance in the coatings field. The use of water instead of organic solvents in two-component polyurethane paints based on polyisocyanates containing free isocyanate groups has not been possible because it is known that isocyanate groups react not only with alcoholic hydroxyl groups, but also with water. In addition, the concentration of active hydrogen atoms emanating from the water in these systems is far higher than the concentration of hydroxyl groups in the organic NCO-reactive component. Therefore, it had to be assumed that, in the ternary system of polyisocyanate, organic polyhydroxyl compound and water, the isocyanate/water reaction predominates with formation of urea and carbon dioxide. This reaction does not lead to cross-linking of the organic polyhydroxyl compounds and results in foaming of the paint mixture through the formation of carbon dioxide.
Although it is known from DE-OS 2 708 442 and from DE-OS 3 529 249 that organic polyisocyanates can be added to aqueous polymer dispersions to improve the property spectrum, these polymer dispersions are not organic polyhydroxyl compounds of the type normally used in polyurethane chemistry as reactants for polyisocyanates. Further, the effect described in these prior publications of adding polyisocyanates to aqueous polymer dispersions is presumably attributable to coating the dispersed polymer with the urea formed from polyisocyanate and water.
It has now surprisingly been found that it is possible by using selected organic polyhydroxyl compounds, i.e. polyhydroxypolyacrylates of the type described in detail hereinafter, as reactants for organic polyisocyanates containing free isocyanate groups to produce aqueous two-component polyurethane coating compositions by emulsifying the polyisocyanates containing free isocyanate groups in the aqueous polymer solution or dispersion. The coating compositions according to the invention have a pot life of several hours and harden to form high-quality, cross-linked films which have comparable properties to known coatings obtained from solvent-containing two-component polyurethane lacquers. This is extremely surprising for the reasons explained above because it had been expected that such a selective reaction would not take place between the polyisocyanate and the hydroxyl-containing polymer when dissolved or dispersed in water.